1. Field of the Invention
The present invention relates to power electronic switching circuits and in particular to voltage converters employing two or more controlled switches.
2. Related Art
Switching circuits operating at high switching speeds and/or slew rates and at high currents are subject to performance degradation caused by parasitic devices, such as the inherent body diode of an active switch. This diode can conduct current even if another current path is available, such as a parallel-connected free-wheeling diode. When a MOSFET switch acting as a synchronous rectifier turns off, the current can transfer from the MOSFET channel into the parasitic body diode. When the control MOSFET turns on the recovery charge stored in the body diode during conduction is swept out. Abrupt reverse recovery of the body diode can cause higher switching losses and high frequency ringing, which places higher stresses on components and can couple to the output and cause noise and Electromagnetic Interference (EMI) associated problems.
Various techniques are available to reduce the effects of body diode conduction. One common method is to add a Schottky diode in parallel with the synchronous rectifier MOSFET and allow enough dead time for the current to transition from the body diode into the Schottky diode. Adaptive and/or predictive gate-drive techniques sense the voltage across the synchronous rectifier MOSFET and adjust the timing delays in order to reduce the time that the body diode conducts, thereby achieving lower losses. Some semiconductor manufacturers make MOSFETs with a tightly integrated Schottky diode in the same package. The tight integration provides a low inductance path with a lower forward voltage drop, which allows the current to transfer directly from the channel to the Schottky diode. Another method exists that employs a magnetic element to produce a current that maintains the body diode in a reverse-biased state.
Unfortunately, many of these methods for reducing body diode conduction and the issues associated with reverse recovery have limitations in their use and applicability. For example, a MOSFET with a tightly integrated Schottky diode may not be available with the desired characteristics (for example, Vds, Rds-On, Qg, package type). If high frequency operation is required, increasing the dead time may not be acceptable. Predictive or adaptive gate-drive may limit the choice of control methods, for example, the designer may need to buy a controller IC from a specific manufacturer or design his or her own digital controller. In addition, a predictive or an adaptive gate-drive may not provide adequate protection against shoot-through or prevent the body diode from conducting. Adding a magnetic element to keep the body diode reverse-biased may increase the complexity to an undesired level.
Accordingly, there is a need to provide a converter power module that effectively controls and minimizes the effect of the body diode. Specifically, there is a need to provide a means to reduce the adverse effects of EMI and power loss due to the parasitic body diode.